A typical transportable shredding machine is a trailer-mounted assembly having a hopper for receiving wood articles, etc., a gasoline or diesel engine-powered comminutor, and a conveyor arranged to transport chips from the comminutor along an upwardly sloping path for discharge into a pile or into the body of a truck or trailer so that they can be hauled away. A typical trailer-type shredding machine is described in U.S. Pat. No. 5,402,950, dated Apr. 4, 1995.
In the shredding machine described in U.S. Pat. No. 5,402,950, the discharge conveyor is pivotable from a position in which it extends upward and to one side of the trailer for discharge of chips, to a nearly upright position for transport. When the discharge conveyor is nearly upright, the center of gravity of the machine, including the trailer, is above a position located centrally between the right and left wheels of the trailer. The conveyor is also positioned inboard of the wheels so that it will not interfere with other vehicles on the highway in turns.
Other transportable shredding machines have discharge conveyors that extend upward and to the rear of the trailer. Some of these rearwardly sloping discharge conveyors are fixed in relation to the trailer. Others are pivotable to a nearly upright position. Still others are designed to fold up and over the trailer. The pivoting or folding of the rearwardly extending conveyors shortens the overall length of the machine for transportation. It also moves the overall center of gravity of the machine, including the trailer, forward somewhat.
A discharge conveyor is, by necessity, heavy, and its weight constitutes a substantial fraction of the overall weight of the trailer and machinery mounted on the trailer. A fixed, rearwardly extending, discharge conveyor, causes the position of the center of gravity of the trailer to be located well toward the rear of the trailer, requiring the wheels to be positioned further toward the rear, and requiring the trailer to be constructed so that the weight forward of the wheels balances the weight to the rear of the wheels so that the trailer exerts a moderate downward force on the trailer hitch of the towing vehicle. The rearward positioning of the wheels makes it more difficult to balance the load on the trailer, impairs the maneuverability of the trailer and towing vehicle, and adversely affects the safety of the trailer and towing vehicle on the highway. Because of the requirement for balance, the overall weight of the trailer, together with the machinery on the trailer, is higher.
The weight of the discharge conveyor will cause the trailer to react excessively to irregularities in the highway surface. Moreover, the discharge conveyor must be securely attached to the trailer to prevent its inertia from causing breakage of fasteners and frame components of the trailer and of the conveyor itself. Moreover, in some case, in the interest of better balance, the gasoline or diesel engine is positioned toward the front of the trailer and remote from the comminutor, requiring special coupling to transmit power from the engine to the comminutor, and increasing the overall length of the trailer.
Even in the case of a rearwardly extending, pivotable, discharge conveyor, the center of gravity is still too far to the rear when the conveyor is pivoted upward. Consequently, problems due to excess weight also exist in these machines and measures must be taken to balance the trailer and machine assembly fore and aft of the trailer axle or axles. Moreover in the case of a pivoting conveyor, whether it be a rearwardly extending conveyor or a sidewardly extending conveyor, additional difficulties arise in preventing breakage due to inertia, and in securing the conveyor against movement relative to the trailer, due to highway irregularities.
Folding conveyors offer a partial solution to the balance problem, but are structurally more complicated than a pivoted conveyor, and often heavier.
In transportable shredders, the weight of the towed shredding equipment, trailer and discharge conveyor is an important consideration, as is the combined weight of the towed equipment and the towing vehicle. A unit weighing up to 12500 pounds can be towed by a heavy ¾ ton pickup truck. However, if the overall weight is greater than 12500 pounds, a heavier towing vehicle is required, which will generally result in a combined weight greater than 26000 pounds.
If the combined weight is 26000 pounds or more, the acquisition cost of the towing vehicle is higher, and the cost of tolls and fuel is higher. A commercial driver's license is also required, and consequently fewer available qualified vehicle operators are available, and they command a higher pay scale. In many cases, it is also difficult to transport the heavier equipment into and away from a job site.